How can I make this all better?!

I’m not sure anyone can help but I need to offload please! (This could be long so I apologise!)

I have 2 DD’s. I love them both equally and unconditionally.

DD1 recently married. She is self disciplined, a perfectionist, ambitious and has a good, albeit stressful career. Due to her job, I haven’t seen her as much as I would like since she left home. I’ve always felt like she doesn’t need me much and that she can cope with anything as she has always been fiercely independent.
If she does ever need me, I’m there for her and she does acknowledge that.

DD2 lives with her DP and has a 1 year old. I see her much more as I help out with the baby. I suspect that she feels as if she has lived in her sisters shadow at times although neither myself nor my ex husband have made her feel this way and have always been proud of what they have both achieved. She qualified in something she enjoys doing and works part time. She is quite anxious and sensitive and I do see her as the more ‘vulnerable’ one.
DD1 picks up on this and ‘jokily’ comments occasionally.
Fast forward to a recent visit to me from DD1...........
After a quick catchup - she burst into tears and told me she’s been feeling very low recently and work arranged some counselling, which she has now finished. Last week she reached the point where she felt like she needed to discuss this with me.
My immediate reaction was to cry with her. She sounded so sad and vulnerable. I felt dreadful as I had no idea
It physically shocked me!
Amongst other issues that she is addressing, she feels that I am far more supportive of DD2 and that DD2 makes underhand comments to her to suggest she is envious of her sisters success.
I have never witnessed this although I know that DD2 does feel that DD1 is unapproachable at times. DD1 wants to ‘chat’ to DD2 about this but knows it’s likely to bring her own insecurities to the surface.
She really doesn’t want to upset her sister - but equally wants to discuss things with her.

They have never fallen out. To anyone else (and me! - until DD1’s visit ) their relationship appears rock solid and enviable.

I do think DD1 has given it a lot of thought and worked out a way to handle it as sensitively as possible but there’s no doubt DD2 will be shocked and upset. She does look up to her big sister and does love her very much.

But I can’t prevent DD1 from approaching her sister as her feelings are just as valid and this is all part of her healing process.

I am so scared they will fall out. I couldn’t bear that.

I am totally drained by this especially as I have never experienced anything like this. My parents and siblings have never had a cross word!

Should I be there when they meet up? Should I just step back and let them work something out between them (praying that after the tears it might make them closer?)

I just want them both to be happy and will do anything to help them achieve that.....

Has anyone experienced anything like this that can reassure me please?